Contact (coupled) sensors based on inductive, piezoelectric or capacitive transducers, are used almost universally in a host of applications ranging from the detection of vibrations in manmade structures, e.g. bridges, to the monitoring of the earth's naturally occurring, or artificially generated, tremors. For example, in seismic exploration for hydrocarbons and minerals, tremors produced by a source of elastic energy are picked up by detectors placed at or near the earth's surface, in water covered areas, on the ocean bottom and in boreholes.
There are two main drawbacks inherent in coupled sensors:                a. The response of a vibrating structure can be altered by the coupled mass of the sensors;        b. The deployment and retrieval of large numbers of sensors is very inefficient and costly. In seismic exploration over land, many hundreds to several thousand sensors, i.e. geophones or accelerometers, are placed on the earth's surface. Each sensor has attached to its body a metal spike which must be driven individually into the ground by hand, to provide good earth-to-transducer coupling. Vast manpower and logistical support are required to place and move such a large number of sensors over exploration areas measuring from hundreds to thousands square kilometers.        
Non-contact (uncoupled) sensors are more suited to applications where transducer coupling effects can alter the response of the vibrating structure to be measured. Vibration measurements made with uncoupled sensors are mostly based on optical waves, electromagnetic microwaves and acoustic ultrasonic waves. The embodiment of these sensors consists of a transducer which emits and directs waves toward the vibrating surface under investigation. The vibrating surface modulates the incident and reflected waves in frequency and phase (Doppler shift). The reflected waves are detected by a receiver sensor and subsequently demodulated to extract the Doppler shift, which is proportional to the velocity of the surface vibrations. Because of the type of measurement performed, these devices are also called velocimeters.
Generally, velocimeters have many inherent drawbacks:                a. Starting with optical sensors, known as Laser Doppler Velocimeters (LDV), they are bulky, expensive, very sensitive to surface roughness and the frequency range of the measured vibrations does not extend below 1 kHz. However, their resolution (the smallest detectable vibration amplitude) can be as high as a few nanometers.        b. Microwave Doppler Velocimeters (MDV) are also bulky and their resolution is several orders of magnitude lower than that obtainable by LDVs. In addition, the accuracy of the Doppler shift measurement is affected by interferences from spurious reflections and other events originating at or just below the surface.        c. Ultrasonic acoustic sensors have found only limited applications in acoustic Doppler vibrometry (ADV) mainly because, compared to LDVs, their resolution is much lower and they lack response to vibrations above approximately 1 kHz.        d. Finally, it is a further very important consideration that all types of uncoupled sensors need to be mounted on an ultra-stable platform, in order to minimize the movement induced in the sensor by ambient noise. Since the surface vibrations are measured relative to the static position of the sensor, sensor movement is one of the major sources of errors in Doppler measurements of surface vibrations. Generally, for experimental set-ups and outdoor applications the platform is provided by a sturdy support, like a rigid purpose built tripod, whereas in more permanent installations, the uncoupled sensor is mounted on a damped large mass or in an anechoic chamber. For very precise measurements inertial platforms are sometimes employed. For use outside of the laboratory and in the field, to obtain for example seismic data measurement, all the above platforms are either too cumbersome or very expensive to implement, or both.        
In order to overcome the drawbacks of coupled sensors, it is therefore desirable to provide an uncoupled sensor which is relatively inexpensive, and which is small and durable enough to be deployed easily in large numbers. In order to be useful in applications where coupled sensors are usually employed, it is desirable for the uncoupled sensor to have sufficient resolution to measure earth surface vibrations, but without being too sensitive to surface roughness. Furthermore, it is desirable to provide an uncoupled sensor which is mounted on a platform which is sufficiently stable to allow the sensor to provide accurate measurements, but is durable and inexpensive enough to be used in large numbers in rough terrain and can withstand rough handling.